It is desirable for a printer to be able to print characters in more than one quality of print, possibly at more than one speed, in order to provide, for example, a high speed of printing with a low quality of printed character and a low speed of printing with a high quality of printed character. Printing at a high speed with a low quality of printed character is commonly performed when a large volume of output is required, as in a data processing environment. On the other hand, in a word processing environment or office environment where a low volume of output is more common, a high quality of printed character is needed, for example in the printing of letters, and any associated reduction in speed is acceptable.
In matrix printers in which each character is formed by printing selected character elements, for example dot elements, in a matrix of print positions arranged in rows and columns, a high speed of printing is achieved by printing only a comparatively small number of character elements resulting in the printing of characters in a low quality. In order to improve the quality of each character additional character elements are printed in selected print positions in the matrix. The positions for the additional character elements are selected so as to provide for enhancement of each character in the horizontal, vertical and diagonal directions by selectively increasing the density of portions of the character. The expression "diagonal direction" is used herein to describe any direction which is at an angle to the horizontal and vertical directions. A printed character can be described as having high or low print quality in the horizontal direction, the vertical direction and the diagonal direction. If a printer is to print characters in more than one quality of print it will be necessary to provide data for printing in each quality. It is expensive to store all this amount of data. In order to save on storage space it is known to provide character printing data for printing each character in only one quality and to derive from this basic data additional data for printing in a higher quality. Higher quality is obtained by selectively increasing the density of the character elements printed.
Known methods of deriving additional data for printing in a higher quality use rules or algorithms which do not provide high quality for all characters and, for some characters, actually result in distortion of the character. Also the rules and algorithms are difficulty to implement in practice.
The object of the present invention is to provide a printer which is capable of printing all required characters in two print qualities without distortion and in which data for printing the characters in a required quality is derived from stored data by an improved method.